This invention relates to a vibration damping apparatus having air springs, linear motors and the like, used as a quadruped support mechanism especially in a semiconductor exposure apparatus.
With the improvement of precision devices such as an electronic microscope and a semiconductor exposure device, there is a need for a high-performance precision vibration-damping apparatus mounting these precision devices. Especially, in the semiconductor exposure device, a table for damping as much as possible extraneous vibration such as vibration transmitted from a setting floor is necessary for appropriate and quick exposure, since an XY stage for exposure must be completely stopped when a semiconductor wafer is exposed. The XY stage is characterized by intermittent operation, referred to as a step & repeat operation, which causes vibration of the table itself. Accordingly, in the table, it is required to implement both vibration damping performances against extraneous vibration and internal vibration caused by the operation of a device mounted on the table.
Regarding this requirement, an active vibration damping apparatus which detects vibration of a table with vibration sensors and imparts driving forces upon a table in accordance with output signals from the sensors, has been introduced into practical use.
FIG. 6 shows the construction of the vibration damping apparatus. A table 8 holding an XY stage 17 is supported by four vibration damping units as a quadruped vibration damping apparatus. The vibration damping units are provided at the four corners of the table 8. The respective vibration damping units have the same construction. Therefore, they have the same reference number with different letter suffixes. A vibration damping unit 1a comprises a horizontal actuator 2a which applies a driving force in a horizontal direction to the table 8, a horizontal acceleration sensor 4a and a horizontal position sensor 5a for detecting horizontal vibration of the table 8, a vertical actuator 3a which applies a driving force in a vertical direction upon the table 8, and a vertical acceleration sensor 6a and a vertical position sensor 7a for detecting vertical vibration of the table 8. In practice, the vibration damping unit 1a may include parts not shown in the figure, for example, a servo valve for supplying air as an operating fluid if the actuators 2a and 3a are air springs, or mechanical springs. However, FIG. 6 illustrates conceptually the construction of the vibration damping apparatus, and it only shows representative parts of essential constituent elements for vibration control of the table 8. As described above, the other vibration damping units 1b to 1d have the same construction. Note that in one of the vibration damping units 1a to 1d, the horizontal and vertical position sensors may be omitted.
As a control method for controlling the active vibration damping apparatus, motion-mode based control is disclosed in Japanese Patent Application Laid-Open No. 7-83276. This control extracts motion modes regarding the accelerations of the table and motion modes regarding the positions of the table from signals from a plurality of acceleration sensors and position sensors, and performs appropriate compensation in each motion mode.
According to this motion-mode based control, six degrees of freedom of rigid-body motion of the table are classified into three horizontal degrees of freedom (two degrees of freedom in the horizontal X- and Y-axial directions, and a rotational degree of freedom around a vertical Z axis), and three vertical degrees of freedom (a degree of freedom in the Z-axial direction, and two degrees of freedom around the X- and Y-axial directions). Then, vibration control is made in accordance with each motion mode.
In the construction of the active vibration damping apparatus shown in FIG. 6, the number of the horizontal actuators 2a to 2d and that of the vertical actuators 3a to 3d are four corresponding to the number of the vibration damping units as support mechanisms. Therefore, there are three degrees of freedom with respect to the horizontal and vertical motion modes, while the number of the actuators is redundant. Accordingly, there are a plurality of ways for vertical motion-mode distributors 11 and 11' and horizontal motion-mode distributors 12 and 12' to distribute motion-mode based table driving forces to the respective actuators.
In the semiconductor-exposure apparatus, the XY stage 17 mounted on the table 8 is characterized by its intermittent step & repeat operation. The repetitive step operation is generally made in the X- or Y-direction of the XY stage 17. Some of the horizontal actuators 2a to 2d which generate driving forces in the step-operation direction receive heavy loads since vibration of the table upon step & repeat operation is suppressed. Considering the structural feature of the table 8, its center of gravity does not always correspond to the central position of arrangement of the vibration damping units 1a to 1d. The table 8 is float-supported by the vibration damping units 1a to 1d for shutting off the vibration table 8 from vibration of the setting floor. In this state, the vertical actuators 3a to 3d constantly receive loads greater than those upon the actuators positioned closer to the center of gravity of the structure (the table 8 and the XY stage 17 in this case). In this manner, the structure and operation of the device mounted on the vibration damping apparatus cause a variety of loads upon the horizontal and vertical actuators.
Since the actuator cannot generate an infinite driving force, an actuator that receives a heavy load cannot generate a driving force corresponding to an output signal from the motion-mode distributor, which causes a so-called actuator-output saturation. If the actuator has a sufficiently large output, such a phenomenon does not occur. However, the artifact supported by the vibration damping apparatus in a semiconductor exposure device generally has a large weight. Further, it is desired that the speed of the step operation of the XY stage be increased so as to improve the throughput of the semiconductor device. Therefore, the table 8 receives a large counter force from the XY-stage driving. If a large-output actuator which has a sufficient performance to meet these conditions is used, the size of the vibration damping apparatus is increased. Then the size of the semiconductor exposure device itself is increased in size and weight. In addition, this is disadvantageous in the point of costs. Accordingly, actuator-output saturation cannot be avoided if loads are concentrated on a part of the actuators.
When actuator-output saturation occurs in the motion-mode based control, a predetermined driving force cannot act upon the table 8 based on each motion mode, which disturbs the motion-mode based control. To realize appropriate and effective motion-mode based control, actuator-output saturation must be avoided. Otherwise, even if the actuator-output saturation does not occur, the concentration of loads upon a part of the actuators should be avoided in consideration of durability. For this purpose, it is necessary to design the motion-mode distributors so as to distribute the table driving force without concentrating loads on a portion of the actuators, utilizing the fact that the horizontal motion modes and vertical motion modes of the table 8 are respectively three, whereas the number of the horizontal actuators 2a to 2d and that of the vertical actuators 3a to 3d are redundant.